Sustainable Solutions in Gas Separation: Exploring the Potential of

Deep Eutectic Solvents

YOUSEF ELHAMARNAH, HAZIM QIBLAWEY

Department of Chemical Engineering

Qatar University

P.O.Box 2713

QATAR

Abstract: In the face of escalating environmental concerns, particularly related to greenhouse gas emissions, this study

delves into the potential of Deep Eutectic Solvents (DESs) as a s ustainable alternative in gas separation

technologies. Focusing on the significant emissions of CO2, SO2, and H2S from industrial processes, this work

reviews the application of DESs for their capture and separation. We investigate the physical properties of DESs,

such as solubility and viscosity, which are crucial for their efficacy as so rbents. This review includes a

comprehensive analysis of various DES formulations, exploring their roles in CO2 absorption, SO2 removal, and

the separation of other gases like H2S. Additionally, we extend our examination to the applicability of DESs in

the oil and gas industry, highlighting their effectiveness in removing sulfur and nitrogen impurities, and their

potential in the extraction of organic constituents. The study reveals that DESs, characterized by their

biodegradability and environmental sustainability, offer promising performance in gas separation, aligning with

the principles of green chemistry. However, challenges such as high viscosity and the need for further

understanding of their solubility dynamics under different conditions are addressed. This work underscores the

importance of DESs as novel sorbents for gas purification and sets a foundation for future research aimed at

enhancing their application on a broader industrial scale.

Key-Words: Deep Eutectic Solvents (DESs), Ionic Liquids Green Chemistry, Sorbent, Carbon capture

Received: April 13, 2024. Revised: September 5, 2024. Accepted: October 9, 2024. Published: November 6, 2024.

1 Introduction

In the contemporary landscape of industrial

processes, the urgent need to address environmental

challenges has become paramount. Among these, the

emission of greenhouse gases such as sulfur oxides

(SOx), nitrogen oxides (NOx), and carbon dioxide

(CO2) from combustion processes stands as a critical

concern, directly contributing to global warming and

climate change. Traditional methods of gas

separation and purification, employing Ionic liquids,

Amino acid salts, and Potassium carbonate, have

been at the forefront of research for the past two

decades[1]–[4]. However, these methods are

increasingly scrutinized for their environmental

footprint, particularly regarding biodegradability,

sustainability, and biocompatibility.

In response to these challenges, this paper introduces

an innovative approach centered around Deep

Eutectic Solvents (DESs). DESs emerge as a

groundbreaking class of solvents, boasting a plethora

of advantages over their traditional counterparts[1],

[5]–[8]. Not only do t hey promise enhanced

environmental compatibility, but they also offer a

range of tunable physical properties that can be

strategically tailored for specific gas separation

processes. This paper endeavors to provide an in-

depth analysis of the physical properties of various

DESs, focusing on their solubility and viscosity,

which are crucial for their efficacy as process

solvents.

Through this comprehensive study, we aim to

illustrate the potential of DESs in transforming the

landscape of gas separation technologies. We will

explore their applications in separating key gases like

SO2, CO2, and H2S from different oil and gas

samples, under varied conditions of temperature and

pressure. The paper also sheds light on the latest

research regarding the role of viscosity in the

practical deployment of these solvents, both at room

and elevated temperatures. By presenting the

thorough advancements of the applications of DESs

in the oil and gas industry, this paper seeks to

underscore their role as the next generation of

sustainable and efficient gas separation agents.

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2 Problem Formulation

Global warming, largely driven by emissions of CO2,

SO2, and H2S from large point sources like fossil-

fueled power stations, poses a significant threat to the

environment. Traditional methods of gas separation

are either inefficient or environmentally detrimental.

Furthermore, the rising need to manage SO2

emissions, which contribute to acid precipitation

affecting human health and infrastructure,

underscores the urgency for more effective solutions.

Similarly, the oil processing industry faces

challenges in removing impurities like sulfur,

nitrogen, and organic compounds from fuels. These

problems demand innovative solutions that are both

effective and sustainable.

3 Problem Solution

The application of DESs in gas separation

processes offers a viable solution. Studies have

shown that DESs, with their tunable properties, can

effectively capture CO2, with the absorption capacity

influenced by factors like molar ratio and gas

pressure. Similarly, DESs have demonstrated

efficiency in SO2 removal under various operational

conditions, proving their potential in managing

emissions. In the context of oil processing, DESs

have been used successfully for the extraction of

sulfur and nitrogen compounds and the separation of

aromatics and aliphatics. These results indicate that

DESs could serve as green and efficient alternatives

to conventional solvents in gas separation and oil

processing applications.

3.1 Application of DESs in gas separation

processes

Several studies have recently emerged on the use of

DESs for gas separation, solubility studies, and

capturing processes for different gasses. The current

literature has mainly focused on emissions from

different large point sources such as the flue gas from

fossil-fueled power stations are the main contribution

to global warming. These gasses mainly consist of

CO2 SO2, and H2S.

3.1.1 CO2 Capture

Studying the CO2 solubility is an important property

for the sequestration of CO2. In recent studies,

different DESs are considered as an emerging

alternative solvents for the CO2 sorption process a

promising green solvent candidates for improving the

enhancement of CO2 capture and also the capture

process cost [9]. According to the conducted

literature survey, the maximum CO2 uptake

performance was influenced by several factors

effecting the DESs. For instance, Altamash et al. [10]

research group studied the effect of increasing the

mole ratio on choline chloride + phenylacetic acid at

(1:2),(1:3), and (1:4) molar ratio for CO2 capturing,

using high-pressure gas absorption at post-

combustion CO2 capture conditions. This group

found a general conclusion for the three mixtures,

that solubility of CO2 increases with increase of the

gas pressure and consequently decreases with

temperature on all the systems. However, an increase

in the molar ratio to 1:3 from 1:2 showed relatively

higher performance in CO2 capture, but lower for 1:4

in comparison with 1:3. Therefore, further studies are

required on the effect of molar ratio for CO2 capture

processing. The effect of molar ratio of a DES was

also studied by Adeyemi et al., Lu et al., and Li et al.

[11]–[13] In a different study by Altamash et al.[1],

four novel choline chloride based DESs with altered

HBDs.were evaluated .The group enhanced their

studied on CO2 solubility in DESs with rheological

characterization on these solvents to furtherly

investigate the bonding interactions between these

NADES. Due to the low viscosity of choline chloride

+ lactic acid, CO2 solubility in it was the best

performing sorbent. Moreover, Deng et al. [14]

considered addressing the effect of alternating the

HBA using five different quarternary ammonium

species for their study on CO2 capture, using a fixed

HBD of levulinic acid at a molar ratio of 1:3. They

elaborated on the influence of the HBA on t he

absorption capacity of the DESs, stating that the

structure of HBA and weak related to that of HBA.

The larger the HBA in the quarternary ammonium

salt, the higher absorption ability of DES for CO2.

3.1.2 SO2 Removal

SO2 emissions has rapidly increased over the past

few years due do the burn in fossil fuels and the

conversion of energy. Its adverse effect is mainly

witnessed as a cause of acid precipitation on the

health of humans and infrastructure. As a result,

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effective SO2 management has been the forefront

research of researchers in fields of gas capturing and

separation.

Yang et al. [15] looked at the SO2 absorption

by tuning different parameters and operational

conditions, such as the temperature, SO2 partial

pressure, and the molar ratio of choline chloride +

glycerol (1:1 – 1:4). They concluded that the

maximum sorption of SO2 was obtained at the lowest

molar ratio (1:1), lowest temperature (20oC), and at

atmospheric pressure, with a value of 0.678 g SO2/ g

absorbent. In another study on the conditions of SO2

absorption, Zhang et al. [16], who studied the

sorption capabilities of Betaine + Ethyl Glycol & L-

carnitine + Ethyl Glycol under the effect of varied

HBA/HBD mole ratios (1:3-1:5) and high

temperatures. They concluded that the mole ratio had

no effect on a bsorption, lower temperature (30oC),

and with the low SO2 partial pressure was favored for

SO2 capturing. This analogy was also sensed in the

study by Deng et al[17]. Moreover, Zhang et al

investigated the potential abilities of regenerating

DES after the SO2 absorption, they have found and

confirmed that SO2 absorption is reversible and with

increase the temperature, regeneration of DESs is

achievable with capacities and stabilities.

Furthermore, Sun et al.[18] also agreed with the

findings of Zhang et al., where Sun et al. confirmed

that all of the four investigated solvents of choline

chloride + urea, choline chloride + ethyl glycol,

choline chloride + malonic acid, choline chloride +

urea are all suitable for sorption reuse and recycling,

after regeneration 100% SO2 molecule within a time

frame of 15mins.

3.2 DESs for separation of other gases

Guo et al. [19] explored the solubility of H2S at

different temperatures ranging from 303.2 – 363.2 K

on tetrabutyl ammonium bromide + caprolactam with

different molar ratios of from 1:1 to 1:7. They found

that the solubility of the 1:1 mole ratio was at

performing the best out of the other molar ratios,

where the highest capacity was 5.4 wt% of at 303.15

K and atmospheric pressure. The effect of

incrementing the amount of HBA: HBD has shown

an adverse effect to the solubility capacitance, as it

decreased with increase HBA: HBD. Moreover, the

regeneration and reusability has also shown

promising capacities of DESs on H 2S. The

absorption process taken place was by the physical

interactions between the DESs and H2S, as the H2S

was noticed to remain in its molecular state after

regeneration of six cycles. Further approaches and

studies on the use of DESs for their use in H2S

capture were only limited to a single paper, as the

lack of literature studies hinder the evaluation of their

field applicability. Other predictions on D ES

absorption for different gases such as CH4, H2, CO,

and N2 were applied on several choline-chloride

based deep eutectic solvents by Xie et al. [20]. A

computational based study by Kamgar et al. [21]

using COMSO-RS and NTRL models to theoretically

evaluate the solubility of CH4, CO, N2, H2.The

general gas solubilities in DESs were reported by

Zhang et al. [22], stating that choline chloride DESs

are usually selective in the order of SO2 > CO2>

CH4>N2>CO>H2. However, the authors also noted

that this trend may differ for certain DES systems.

3.3 Applications of DESs in oil processing

3.3.1 Desulphurization of fuel oils

(Removal of mercaptans)

The extraction of mercaptans from fuels has become

a frontier topic in research that requires special

attention. DESs have shown to become an emerging

route for their application in desulfurization of

different fuels with different sulfur impurity. Li et al.

[23] Reported the use of tetrabutyl ammonium

chloride-based DESs with the use of a series of well-

recognized HBA: glycerol, ethylene glycol,

propionate, and malonic acid to extract

Benzothiophene from a n-octane based fuel matrix.

The maximum extraction in their study has shown

higher removal rates than conventional ILs ones. In

one cycle, the 82.23%. In another study by Li et

al.[24] , The highest removal efficiency per cycle was

recorded, a maximum removal efficiency of 89.53%.

Their study involved enhancing the DES mixture

with a metal ion of FeCl3. The group explained the

effect of the metal ion on the extraction mechanism

by exploring the interactions from IR spectra. The

FT-IT peaks have shown that the interactions

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between the DESs hydrogen bond and the metal ion

were weak, yet almost destroyed. However, the

hydrogen bonding interactions between the DESs

mixture and the sulphur impurity of

dibenzothiophene was enhanced gradually, resulting

in a network breakdown between the intramolecular

forces between the DESs itself. Therefore, improving

and extraction process. Tang et al. [25] suggested two

possible explanations on the extraction excellence in

DESs (tetrabutylammonium bromide + sulfolane)

with thiophenic sulfur in a heptane based fuel. The

first factor is the  −  interactions between the

aromatic functional group in the DES and the

thiopheic sulphur. The second reason is the

complexation of the thiphenic sulfur with the HBD in

the DES system. The mechanism of DES/sulphur

extraction was also represented by the Tang et al.,

shown by Figure 1.

Figure 1: The extraction process of sulphur from an oil

mixture using DESs [25]

3.3.2 Denitrification of fuel oil

Ali et al. [26] were the first to report the experimental

study on the use of choline chloride + phenylacetic

acid using a t he molar ratios of 1:1 and 1:2 as an

extractants for a promising denitrification technique

from fuels. Among these two molar ratios, the 1:2

showed superior performance in denitrifying basic

and non-basic nitrogen-based compounds. Pyridine

and carbazole nitrogen compounds were separated

from an n-heptane fuel sample by physical extraction

at ambient temperature and atmospheric pressure.

The removal efficiencies of pyridine and carbazole

were 99.2% and 98.2%, respectively. Ali et al. group

had also mentioned that the extraction efficiency was

not affected by the choline chloride + phenylacetic

acid, even after four sets of regeneration, oil mass

ratios, and elevated temperatures. A screening of 94

sets of different DESs using COMSO-RS and NTRL

correlations was reported by Hizaddin et al.[27] for

the predication of potential extraction candidates of

nitrogen from diesel. Accordingly, an experimental

approach was conducted on a n ammonium and

phosphonium hydrogen bond a cceptor based based

DESs at 1:2 molar ratio based on t he preliminary

study. The experimental data had come into an

agreement with the experimental data by using the

NTRL model with an root mean square value of 0.6%

However, no nitrogen removal efficiencies were

recorded in this study. The use of 1H NMR was used

to access compositional analysis of the nitrogen in the

experimental studies [28] .

3.3.3 Removal of BTEX

The removal of aromatics and aliphatic compounds

is a co mplicated process in the petrochemical

industry. The minimal difference in boiling points

between the extract and the extractant hinder the

process of BTEX removal from gas streams.

Moreover, the formation of azeotrope combinations

is also a challenge in their removal. The use of DESs

in the separation of different BTEX.

Rodriguez et al. [29] compared the extraction of a

hexane + benzene mixture using ammonium based

DESs with glycerol and ethylene glycol with of mole

ratio (1:2) with the extraction performance of

sulfolane. After characterizing both solvent’s

viscosity and density, the ethylene glycol based deep

eutectic solvent was favored glycerol, due to the

lower magnitudes in viscosity and density that

enhanced the extraction of benzene. The liquid-liquid

equilibria (LLE) was determined at 298,308 K and at

atmospheric pressure. The solute distribution

coefficients of the solfolane were higher than DESs,

and the selectivity values were also higher for

solfolane. Moreover, the group of researchers

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apprised to that the performance of DESs in

extraction was comparable to ILs and may be

considered as the new generation of solvents.

Mukhtar et al. [30] studied the extraction of an

aromatic/aliphatic mixture from a n aptha steam

cracking unit a series of phophonium based DESs at

Methyltriphinylphosphonuim bromide + ethylene

glycol with three different molar ratios of 1:4,1:6, 1:8

at elevated temperatures of 300,308,318 K. The

authors described their satisfaction with the

performance of the 1:6 molar ratio DESs in

comparison with sulfolane. Moreover, the authors

also compared the study with N-formylmorpholine,

stating that the 1:4 DESs showed better separation in

comparison with it. The effective extraction

temperature was recorded at 318 K. Furthermore, a

complementary study by Mukhtar et al. [31].

discussed the removal of aromatics from ethylene

cracking unit using LLE using two novel

DESs: tetrabutylphosphonium bromide + ethylene

glycol and tetrabutylphosphonium bromide +

sulfolane. This study altered the HBD molar ratio of

1:4,1:6,1:8 and operated at temperatures of

313,323,333 K. This group suggested that more

investigation on this particular group requires further

investigation prior applying and using these two

DESs in the separation of aromatics.

Gonzalez et al. [32] investigated the use of two

choline chloride based DESs:lactic acid and glycerol

with molar ratios of 1:2 on LLE for the purpose of

extracting benzene and acetate mixtures from a

hexane based fuel model at 298,308,318 K. In their

comparison with ILs, both choline chloride based

DESs have shown higher selectivity and showed no

signs of decomposition with time. The favored

temperature of the extraction process was at 308 K.

They concluded that the choline chloride + glycerol

DESs showed the best separation at room

temperature and lactic acid was more soluble to

benzene.

4 Conclusion

This comprehensive review has underscored the

pivotal role of Deep Eutectic Solvents (DESs) in

revolutionizing gas separation technologies and

addressing environmental challenges associated with

industrial emissions. Our exploration revealed that

DESs are not only effective in capturing and

separating key greenhouse gases like CO2, SO2, and

H2S but also hold significant potential in various

applications within the oil and gas industry. The

studies reviewed highlight the advantageous physical

properties of DESs, particularly in terms of solubility

and viscosity, which are critical for their performance

as novel sorbents.

The research into CO2 absorption, SO2 removal, and

H2S separation using DESs has demonstrated their

capacity for efficient gas capture under a range of

conditions, emphasizing their versatility.

Furthermore, the application of DESs in the removal

of sulfur and nitrogen impurities, as well as the

extraction of organic compounds from oil samples,

positions them as a p romising green alternative to

conventional solvents.

However, this review also identifies key challenges

that need to be addressed to fully harness the potential

of DESs. The high viscosity of some DES

formulations and the need for a deeper understanding

of their solubility and polarity dynamics are areas

requiring further research. Additionally, while

laboratory and bench-scale studies show promising

results, scaling up these applications to industrial

levels remains a critical step for the future.

In conclusion, DESs emerge as a beacon of hope in

the quest for sustainable industrial processes. As we

continue to seek solutions that align with

environmental sustainability and green chemistry

principles, DESs offer a promising path forward.

Their adaptability and efficiency in gas separation

and purification, combined with their eco-friendly

properties, pave the way for their expanded use and

development in future industrial applications.

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Contribution of individual authors to

the creation of a scientific article

(ghostwriting policy)

Yousef Elhamarnah: Data collection, Writing

manuscript, methodology.

Hazim Qiblawey: Conceptualization, methodology,

Review and editing.

This study was made possible by a g rant from the

Qatar National Research Fund [GSRA7-1-0602-

20142], which was awarded by (QNRF, a member of

Qatar Foundation). The authors are entirely

responsible for the contents of this document.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

Conflict of Interest

The authors have no conflicts of interest to declare

that are relevant to the content of this article.

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(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the

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EARTH SCIENCES AND HUMAN CONSTRUCTIONS

DOI: 10.37394/232024.2024.4.14

Yousef Elhamarnah, Hazim Qiblawey

E-ISSN: 2944-9006

122

Volume 4, 2024